data-saving: implement gate with bypass

- Implement DataSavingGate util (previous commit was only the shell)
- Update ProfilePictureWidget to always load avatars via UniversalImage
  using fileId, bypassing CloudFileWidget and its data-saving check
- Keep larger media under data-saving control
- Add i18n strings for data-saving mode

Signed-off-by: Texas0295 <kimura@texas0295.top>

assets/i18n/zh-CN.json

@@ -316,6 +316,7 @@
   "settingsRealmCompactView": "紧凑领域视图",
   "settingsMixedFeed": "混合动态",
   "settingsDataSavingMode": "流量节省模式",
+  "dataSavingHint": "流量节省模式",
   "settingsAutoTranslate": "自动翻译",
   "settingsHideBottomNav": "隐藏底部导航",
   "settingsSoundEffects": "音效",